The present invention relates to a borehole opener, particularly for enlarging this hole underneath a casing in the field of oil prospecting, the hole opener comprising a body of longitudinal axis, a duct for drilling fluid, formed longitudinally in the body, and at least two hole-opening arms which have an active part equipped with cutting means, which are distributed symmetrically in the body about the longitudinal axis, and which are arranged therein in such a way that they can be moved between a position of rest in the body and an active position partially out of the body.